The present invention relates generally to a method for enhancing seismic data and more particularly to a method for attenuating multiple reflection events in seismic data.
In seismic prospecting, it is conventional to place a plurality of seismic receivers along the earth's surface at spaced locations. A plurality of seismic sources disposed at spaced locations along the earth's surface can then be activated to generate seismic waves which propagate outwardly in all directions. Vibrating devices, explosive devices and impulsive devices are all exemplary of such seismic sources. The seismic waves this generated are reflected, refracted and diffracted from subsurface formation interfaces, and some of these diverted seismic waves are detected by the seismic receivers and can be processed to form a seismic signal. Such seismic signals can be displayed as seismic sections which contain information about the time, duration and intensity of the diverted seismic waves. Seismic sections can be studied to extrapolate information regarding the type and location of subsurface formations producing the diverted seismic waves. This information can, in turn, be employed to evaluate the subsurface formations for oil- and gas-bearing properties.
Seismic energy which has generally been reflected only once from a reflecting subsurface interface is commonly referred to as a primary, whereas, seismic energy which has been reflected more than once from a reflecting subsurface interface is commonly referred to as a multiple. Such reverberating seismic energy can produce multiple or secondary reflection events in the seismic data from one or more reflecting interfaces in the earth. Consequently, the presence of multiple reflection events or more simply, multiples, in the seismic data can result in confusing and, oftentimes, noninterpretable seismic data.
One method for suppressing multiple reflection events in seismic data is to sort the seismic data into common depth point (CDP) gathers of seismic signals and then normal moveout correct and sum the moveout corrected seismic signals of the CDP gather. The utility of this technique depends upon differences in the moveout velocities for primary and multiple reflection events so as to cause primary reflection events to be reinforced and the otherwise coherent seismic energy represented by multiple reflection events to become less coherent and thus amenable to attenuation by summation of the moveout corrected seismic signals in a CDP gather. However, once this technique has been applied to a CDP gather of seismic signals, further repetitions are not possible because one cannot "unstack" the stacked signals. Moreover, stacking the seismic data precludes the use of recently developed seismic data processing techniques for evaluating range dependent amplitude (RDA) variations as set forth by Bodine, et al., in U.S. Pat. No. 4,646,239.
Alternatively, J. Ryu described in "Decomposition of Seismic Gathers into Velocity Components by a Space-Time Filter," 50th Annual SEG International Meeting (Nov. 17-21, 1980), a method for identifying and isolating various reflection events in common depth point gathers of seismic data to remove unwanted reflection events. In particular, Ryu describes overcorrecting primary reflection events and undercorrecting multiple reflection events for normal moveout to first separate multiples from primary reflection events and then employing a two-dimensional (space time) filter to suppress the multiples. Such technique, however, is dependent upon differences in normal moveout velocities for multiples and primary reflection events.
The present invention provides a novel method for attenuating multiple reflection events in seismic data which overcomes the shortcomings of the above-described techniques. More particularly, the present invention sorts the seismic data into common endpoint gathers whereby differences in normal moveout velocities for primary and multiple reflection events as well as differences in structural dip for primary and multiple reflection events can be employed to suppress multiples. Additionally, since the present invention does not depend upon summation techniques to attenuate multiples, the process can be repeated as often as desired to suppress additional multiples having differences in normal moveout velocity and/or structural dip.